As a conventional flow meter, there is, for example, that which is disclosed in Japanese Patent 3630916, which is hereby incorporated by reference in its entirety. This flow meter is provided with a flow path block that has a main tube path (a main flow path) that connects to a pipe wherein a fluid to be measured flows, where a pressure difference due to a narrowing mechanism in the main tube path is used to branch the flow of the fluid to be measured to a branch flow path that is connected to the main tube path. A thermal flow rate sensor for detecting a thermal shift that depends on the flow of the fluid to be measured is provided in the branch flow path, making it possible to measure the fluid to be measured down to the range of extremely small flow rates.
The correspondence between the flow in the branch flow path and the flow rate of the fluid to be measured that is flowing in the main flow path is used in this conventional flow meter. For example, as described in Japanese Patent 2517401, which is hereby incorporated by reference in its entirety, calibration of the flow rate sensor that is disposed in the branch flow path is performed to produce a correspondence curve that indicates the relationship between the flow rate of the fluid to be measured and the sensor signal value, where an approximation formula for the calibration curve is calculated using the least squares method, or the like. The flow rate of the fluid to be measured is calculated using the approximation formula when measuring the flow rate of the fluid to be measured, flowing in the main flow path.
Because, in the conventional flow meter, the flow rate is measured using an approximation formula that expresses the relationship between a sensor signal value of the flow rate sensor and the flow rate of the fluid to be measured that is flowing in the main flow path, this approach is susceptible to the effects of variability in the sensor signal value due to differences between the flow rate sensors, and thus there is the problem of having to do unit-by-unit adjustments when sensors are changed, which cannot be performed easily. FIG. 10 is a diagram illustrating the relationship between the signal value for the flow rate sensor disposed on the branch flow side and the flow rate of the fluid to be measured that is flowing in the main flow path. As illustrated in FIG. 10, with a conventional flow meter, the fluid to be measured is caused to flow in the main flow path at, for example, 50, 100, and 200 (L/min) as reference flow rates, and the flow rates of the fluid to be measured are measured for each using the flow rate sensor on the branch flow side at each of these reference flow rates, to obtain sensor signal values a1, a2, and a3. These measured values are used to calculate an approximation formula a of a calibration curve that expresses the relationship between the flow rate of the fluid to be measured that flows through the main flow path and the sensor signal value from the flow rate sensor, and the relationship between the flow of the fluid to be measured in the branch flow path and the flow rate of the fluid to be measured that flows in the main flow path is specified by this approximation formula a.
However, when it is necessary to change the flow sensor due to a failure, or the like, and the flow rate sensor is simply exchanged without performing any type of adjustments, there will be variability in the sensor signal values, due to differences between the flow rate sensor units, even if the flow rate of the fluid to be measured, flowing in the branch flow side, is the same as before the flow rate sensor was exchanged. Because of this, when a sensor is changed in a conventional flow meter, it is necessary to readjust the relationship between the sensor signal value and the flow rate of the fluid to be measured that is flowing in the main flow path in accordance with the sensor after the swap because the sensor signal values will go to b1 and c1, given the sensor after the fit swap, when the flow rate of the fluid to be measured is at the 50 L/min standard flow rate, for example, as illustrated in FIG. 10.
In this case, calibration must be performed as described above on the flow sensor after the swap to calculate the approximation formula (the approximation formulas b and c in the example illustrated in FIG. 10) for the calibration curves that show the relationships between the sensor signal values and the flow rates of the fluid to be measured that flows in the main flow path, requiring the same calibration as at the time of shipping.
Additionally, in the conventional method for calibrating a flow meter, a device is required in order to measure the reference flow rate of the fluid to be measured, flowing in the main flow path, each time there is an adjustment, and it is difficult to perform the adjustment operation in an installation location that does not have this device. Because of this, it is necessary to remove the flow meter each time the sensor is swapped, in order to carry the flow meter from the installation location to a workshop, or the like, having the aforementioned device in order to perform the adjustment, and in some cases the flow rate measurements are not performed in the piping facilities during the period of the adjustment.